The invention concerns a method for controlling the electrical system of a motor vehicle.
The number of electrical loads in motor vehicles continues to increase. Even today, electrical loads, like starter, headlights and interior lighting, radio and cell phone, power windows and air conditioning, outside mirror adjustment and seat adjustment, as well as central locking, are standard equipment in a vehicle, and it is foreseeable that additional electrical loads will be added in future vehicles. These loads must all be supplied with electrical power.
For power supply to electrical loads, an electrical system is present in each motor vehicle, to which, on one side, all electrical loads and, on the other side, generally a generator and a battery are connected as a power source. While the generator is driven by the internal combustion engine of the vehicle and provides electrical power when the engine is running, the system battery serves as additional power storage that is mostly loaded during the starting process of the internal combustion engine, but which also serves as a buffer when the generator alone is not capable of providing the required electrical power. This is the case, for example, at low generator speed and when a number of loads are connected.
The generator and system battery must be designed accordingly to maintain power supply to these loads.
In order to cushion short-term requirements on a power supply without having to design the generator and battery larger on this account, an additional charge accumulator is proposed in German patent document DE 198 59 036. In the electrical system for a motor vehicle according to DE 198 59 036, a secondary system is connected in parallel to the primary system, including a generator, a system battery and at least one load, the secondary system having a controllable DC/DC converter connected to a supercapacitor. The supercapacitor, depending on the operating state of the vehicle, is charged or discharged. Under unfavorable operating conditions, the power stored in the supercapacitor can be made available to the electrical system. The load on the system battery is reduced on this account.
A shortcoming in this prior art consists, among other things, of the fact that the emphasis is on recuperatively produced power, i.e., power recovered when the vehicle is braked by the generator. If, however, too many loads depend on the electrical system, overloading of the generator can occur.
The task of the present invention is to provide a method for controlling an electrical system for a motor vehicle, with which it is ensured that the generator is not overloaded.
The solution of the above task according to the present invention is based on the finding of the inventor that undersupply of the electrical system generally occurs when the generator operates at low speeds. A low speed, especially in idle, is frequent during cold starting in winter or during so-called stop-and-go operation with the air conditioning engaged in summer. On the electrical side, the generator is then up to 100% loaded, since heaters are switched on in the winter and additional loads are connected. These can no longer be supplied at low speeds of the generator. The battery assumes supply or individual loads, which, under some circumstances, can no longer be supplied and must necessarily be removed from the electrical system, in order to avoid interruption of power supply.
During an increase in speed, for example, during starting, the generator also remains 100% loaded, but now it can cover the entire demand for electrical power, i.e., switched-off loads can now be gradually switched on. When the speed rises still further, electrical power is generated that is no longer directly required, i.e., excess power is available. This excess power can now be used, in the first place, to charge the battery and, in the second place, stored for later use in the storage capacitance. In a subsequent idle phase, the storage capacitor is discharged again. This prevents the battery from being loaded in cycles.
The method according to the invention controls an electrical system of a motor vehicle with a generator, a battery to supply the electrical system, at least one load and a storage capacitance for brief furnishing of electrical power as required, during which the battery is charged when the generator produces excess electrical power. According to the method, at least one load is supplied and the battery and the storage capacitor are charged by the generator in load operation or during fuel cutoff in the overrun. The at least one load is additionally supplied by the battery and/or the storage capacitor when the instantaneous generator is not sufficient. An instantaneous required electrical load power in load operation or fuel cutoff is recorded, and the load power with a no-load power of the generator and charging of the storage capacitor by the generator when the no-load power of the generator is less than the load power during load operation or fuel cutoff in the overrun is compared.
In an exemplary embodiment of the invention, one, or if technically possible, several of the following features are also implemented:                the storage capacitor is charged by the generator when the no-load power of the generator is smaller than the load power by a stipulated amount;        the storage capacitor is charged by the generator during fuel cutoff;        both the storage capacitor and the battery are charged by the generator when the power that can be maximally transferred from the generator to the storage capacitor is less than the difference between the instantaneous power of the generator and the load power;        follow-up loads and standby loads are supplied after engine stop by the storage capacitor;        the battery is charged by the storage capacitor after engine stop;        the battery is charged by the storage capacitor when its charge state drops below a threshold value and excess charge is available in the storage capacitor;        the process is automatically restarted at stipulated time intervals.        
The solution according to the invention has the following advantages, among others. The charge balance is improved with a simultaneous increase in battery lifetime, since fewer charging/discharge cycles must be accomplished by the battery. The battery size and generator size can optionally be reduced as a function of the under-coverage frequency, the loads and the size of the storage capacitor and the DC/DC converter.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.